TUNIS, Tunisia  The Islamist party dominating Tunisia's ruling coalition Thursday rejected its own prime minister's decision to replace the government to try to appease critics.

A few dozen protesters also tried to demonstrate in front of the Interior Ministry but were driven off by tear gas, avoiding an apparent attempt to reprise the riots that convulsed the heart of the capital Wednesday.

The announcement by Ennahda throws into question efforts to resolve one of the worst crises Tunisia has faced since its revolution two years ago and makes plain that there are divisions not just between the Islamist government and the largely secular opposition, but within the ruling party itself.

The country's main labor union also declared a general strike for Friday over the assassination, a provocative move that will shut the country down and is expected to inflame tensions in a country already on edge after Chokri Belaid, a fierce government critic, was shot several times in his car just outside his home Wednesday morning by unknown assailants.

Though the capital was otherwise mostly calm Thursday, there were full-scale riots in the southern mining city of Gafsa, where Belaid's Popular Front coalition of leftist parties has a great deal of support.